Wafer sensing and clamping monitor

Abstract: A wafer position and clamp sensor. A circuit 114 monitors capacitance between two electrodes 22,24 within a wafer support 14,16,18,20. With no wafer 12 on the support, the capacitance falls in one range, with the wafer in place but not clamped, the capacitance falls in a second range and with the wafer held in place by an electrostatic attraction the capacitance falls in a third range. The sensed capacitance is converted to a frequency and then a DC voltage level 164 that can easily be sensed and used to confirm wafer placement and then confirm wafer clamping.

Description: Field of the Invention The present invention relates to securing a semiconductor wafer against a wafer support in an ion implantation system. Background Art U.S. Patent No. 5,103,367, entitled Electrostatic Chuck Using AC Field Excitation relates to a mechanism for holding semiconductor wafers in contact with a support during treatment of the wafers. The '367 patent discloses three electrodes, two of which define a substantially planar surface and are embedded within a thin dielectric film. These two electrodes are excited by a low-frequency AC supply to produce sine wave fields of controlled amplitude and phase. The third electrode provides a reference point for the other two electrodes. By controlled rates of voltage application and removal, low-voltage gradients are obtained on the wafer support. This results in no retentive forces between the dielectric medium and the wafer. A low alternating current amplitude excitation of the chuck enables capacitative current sensing of the relative positions of the wafers and the dielectric film enabling simple control of voltage application to the electrodes. Disclosure of the Invention In one aspect, the present invention provides the method, defined by claim 1, for controlling ion implantation of a semiconductor wafer that is electrostatically attracted to a wafer support prior to implantation. In another aspect, the present invention provides the apparatus, defined by claim 5, for securing a semiconductor wafer to a wafer support in an ion implantation system. The preambles of claims 1 and 5 reflect the state of the art according to the aforementioned U.S. Patent No. 5,103,367. According to the invention, the presence of the semiconductor wafer on the support is sensed by measuring the capacitance between two electrodes attached to the support. Once the measured capacitance between the two electrodes reaches a value indicating a wafer has been placed on the support, the wafer is secured to the support by creating an electrostatic attraction between the wafer and the support. A change in capacitance between the two electrodes as the electrostatic attraction is created is used as a check to make sure the wafer has been secured to the support by the electrostatic attraction. The capacitance continues to be monitored, and ion implantation of the wafer is suspended if the measurement indicates that the wafer is not properly secured to the support. Brief Description of the Drawings Figure 1 is a schematic showing a power supply, electrostatic clamp assembly and capacitance measuring circuit; Figure 2 is a plan view of a wafer support used in an ion implanter; Figure 3 is a view as seen from the plane 3-3 in Figure 2; Figures 4 and 4A are schematics of a capacitance sensing circuit; Figure 5 is a power supply circuit for energizing the sensing circuit of Figures 4 and 4A; and Figure 6 is a schematic of a control system that utilizes an output of the capacitance sensing circuit of Figures 4 and 4A to control an ion implanter. Description of the Embodiment The drawings illustrate a clamp assembly 10 for supporting and holding a semiconductor wafer 12 (Fig. 3) for processing. The clamp assembly 10 includes a backing plate 14 which is preferably formed of alumina or molybdenum, a base member 16 also formed of alumina, a dielectric layer 18 of glass, and a dielectric layer 20 of alumina. Electrodes 22, 24 are interposed between the glass layer 18 and the dielectric layer 20 and a heating element 26 is interposed between the glass layer and the base member 16. A gas fitting 28 extends through the chuck assembly opening into the interface between the wafer and the layer 20 to provide gas conduction cooling between the wafer and the chuck as described in U.S. Patent No. 4,261,762. A gas distribution groove 29 is formed in the top surface of layer 20 to aid in the distribution of the gas. The base member 16 defines a manifold having a channel 30 for the flow of a cooling fluid. In the preferred embodiment shown, the channel is formed as a spiral; however, it can also follow a meandering path or it can be a series of interconnected channels. The channels are closed to define an enclosed conduit or conduits by means of the backing plate 14, which is sealed against the manifold. Openings are provided in the backing plate for a coolant inlet fitting 32 and a coolant outlet fitting 34. Since the present chuck assembly is intended to perform under a wide range of temperature conditions, the cooling medium flowing through the manifold can be either a liquid or a gas, depending on the application. Clamp Construction The dielectric layer 20 is preferably formed of a thin (about .25 mm) layer of high purity (99.5%) alumina. The electrodes 22 and 24 are then formed on the bottom (as seen in Fig.3) surface of the dielectric layer, preferably by screen printing a paste of powdered copper aluminum or silver palladium metals and glass frit onto the dielectric layer and then firing it at about 700°C. As shown in Figure 2, the electrodes are essentially half circular in plan view. The heating element 26 is formed by screen printing a paste of powdered tungsten and glass frit onto the manifold in the form of a continuous meander, the geometry of the meander preferably being arranged to provide a higher heater power density at the outer edge to optimize temperature uniformity across the chuck. Once the electrodes 22, 24 and heating element 26 are fired onto the dielectric layer, the layer 20 is bonded to the manifold. After the above assembly is complete, the backing plate 14 is furnace brazed or sealed by means of a sealing glass to the bottom of the manifold 16. As shown schematically in Figure 3, access holes 36, 38 are formed through the backing plate 14, the manifold 16 and the dielectric layer 18 for a first conductor 40 connected to the electrode 22 and for a second conductor 42 connected to the electrode 24. The conductors 40, 42 are attached to the electrodes by brazing or other convenient methods such as by providing spring contacts engageable with the electrodes and are connected to a switching power supply 44 (Fig. 1) which provides a signal of about 3 kilovolts DC to create the electrostatic clamping force to a semiconductor wafer 12 placed on the surface of the dielectric layer 20. Access holes 46, 48 are also formed through the backing plate 14 and the manifold 16 for a third conductor 50 attached to one terminal of heating element 26 also by brazing or the like, and a fourth conductor 52 similarly attached to the other terminal of heating element 26 to connect the heating element to a second power source 54, typically operating at 120 volts. Preferably, the access holes 36, 38 and the hole for the gas fitting 28 are machined into the structure with the holes 36, 38 sealed with sealing glass and the fitting bonded into its hole with sealing glass. Capacitance Sensing Circuit As seen in Figure 1, two inputs 110, 112 from the electrodes 22, 24 are coupled to a capacitive sense circuit 114. A capacitance across these two inputs 110, 112 corresponds to the capacitance between the electrodes 22, 24 and is influenced by the presence of a wafer as well as the voltage applied to the electrodes. As shown in Figure 4, these two inputs are coupled to an operational amplifier 120 within an integrated circuit 122. The integrated circuit is a commercially available circuit designated LF 356 and can be obtained from National Semiconductor. The operational amplifier 120 generates an output which oscillates with a frequency directly related to the capacitance between the inputs 110, 112. The oscillating output signal from the operational amplifier 120 varies between plus and minus 9 volts. This signal is rectified and shaped and then coupled to a comparator amplifier 130 having a reference input 132 defined by a zener diode 134 at 3 volts. The comparator amplifier 130 creates a square wave signal output having a fixed 10 microsecond ON period whose frequency varies depending on the sensed capacitance. For a 4-inch circular diameter wafer, this period is approximately 20 microseconds with no wafer in place, 30 microseconds with a wafer placed on the layer 20, and 40 microseconds with the wafer in place and a clamping voltage (approximately 3 kilovolts) applied to the electrodes by the power supply 44. An output from the comparator amplifier 130 turns on and off light-emitting diodes 140, 142 which are optoisolated from corresponding photodetectors. A top photodetector 144 is used for diagnostic purposes and an output 148 from the detector 144 can be coupled to an oscilloscope, for example, for monitoring frequency changes with capacitance. A second photodetector 146 generates a signal which turns on and off a transistor 150 which is coupled to an analog switch 152 (Fig. 4A). This analog switch 152 has an input (IN) coupled to the collector of the transistor 150. As the transistor turns on and off, outputs (S1, S2) of the switch 152 sequentially change state from ground to 8 volts in accordance with the square wave frequency output from the comparator 130. The output from the analog switch 152 is integrated by a resistor, capacitor circuit 154 so that the input to a non-inverting input 160 of an operational amplifier 162 is a voltage level directly related to the capacitance sensed by the circuit 114. This operational amplifier 162 operates as a voltage follower so that an output 164 designated FVOUT is a DC output signal directly related to sensed capacitance. This DC output signal is used by an implanter control system 250 (Fig. 6) to monitor performance of the system. The wafer handler for depositing a wafer onto the chuck is activated in response to an appropriate no wafer condition being sensed. Once the wafer is sensed, an output from the control circuit 250 activates the DC power supply 44 to energize the electrodes 22, 24 causing an electrostatic attraction between the clamp and the wafer. Turning to Figure 5, this figure discloses a power supply circuit 200 for generating voltages appropriate for the capacitance monitoring circuit 114. Two inputs 210, 212 at the left of Figure 5 provide a 12 volt signal which energizes a light-emitting diode 213. The 12-volt signal is then coupled to integrated circuit voltage regulators 216, 218 for producing +8 and +5 volts. A DC to DC converter 220 provides plus and minus 15-volt signals. These voltages are applied to the circuitry of Figures 4, 4A for providing the capacitance monitoring capability of the present invention. Operation In operation, a wafer 12, which is to be held within an ion beam for processing, is placed upon the surface of the layer 20, and the supply 44 will be energized to apply an electrostatic attractive force between the wafer and the layer 20 sufficient to maintain the wafer in position on the chuck. The chuck 10 can then be both rotated and translated to bring the wafer into the ion implantation path of the ion beam. The implanter control system 250 (Fig. 6) has a large number of device interfaces which receive inputs from sensors 252 (for example, gauges measuring pressures, voltmeters, encoders measuring mechanical position, and the FVOUT output 164), and sends operating commands to mechanical and electrical components 254 (for example, valves, power supplies, robots, and the electrostatic clamp power supply 44). Within the control system there are programmed a variety of cross checks which must be performed before sending an operating command (for example, the valve to a cryopump cannot be opened unless it is verified that the chamber is already under rough vacuum). The output 164 indicates the wafer's presence and whether it is clamped and provides critical information for these cross checks to implement wafer handling and implantation operations. For example, the assembly 10 would not be rotated into a vertical position unless the wafer was verified to be securely clamped. Similarly, the clamp would not be activated unless the wafer's presence on the clamp was verified. When a critical cross check fails, the control system 250 has the ability to place the implanter into HOLD (suspend further operation) in order to avoid damage to the machine or to produced wafers. A second function of the series of cross checks is to provide an operator interface 260 information about machine status. Specifically, when a cross check fails, an alarm message is generated for display on an operator interface screen, and is recorded in a data log maintained on a disk drive. This information permits an operator to take corrective action to restore normal machine operation. Figure 6 represents current state-of-the-art for equipment of this type, and the addition of the capacitive sensing circuit 114 represents an advance in the quality of information available about the status of the wafer on the clamp 10. While the preferred embodiment of the invention has been described with a degree of particularity, it is the intent that the invention include all modifications and alterations from the disclosed design falling within the scope of the appended claims.

Claims: A method for controlling ion implantation of a semiconductor wafer (12) that is electrostatically attracted to a wafer support (10) prior to implantation, the method comprising: sensing placement of the semiconductor wafer (12) on the wafer support (10) using two electrodes (22,24) attached to the support; once the placement of the wafer on the support has been sensed in the sensing step, securing the wafer to the support by energizing the two electrodes to create an electrostatic attraction between the wafer and the support; characterized: in that the sensing step comprises providing a capacitance signal related to a capacitance between the two electrodes (22,24) and determining that the capacitance signal has reached a value indicating that a wafer has been placed on the support; by monitoring a change in the capacitance signal related to the capacitance between the two electrodes as the electrostatic attraction is created, and thereafter, to assure the wafer has been, and continues to be, properly secured to the support by the electrostatic attraction; and by suspending ion implantation of the wafer in the event that the capacitance signal indicates that the wafer is not properly secured to the support. A method according to claim 1, wherein the step of securing the wafer (12) comprises applying a direct current voltage between the two electrodes (22,24). A method according to claim 1 or claim 2, wherein the monitoring step includes generating a warning message and suspending movement of the wafer support (10) to an implant position, if proper securement of the wafer to the support is not sensed. A method according to any of claims 1 to 3, wherein the step of providing a capacitance signal comprises creating a direct current signal (FVOUT) related to the capacitance between the two electrodes (22,24). Apparatus for securing a semiconductor wafer (12) to a wafer support (10) in an ion implantation system, the apparatus comprising: a wafer support (10) including two electrodes (22,24) for attracting a wafer (12) to the wafer support by means of an electrostatic attraction; a power supply (44, 254) coupled to the two electrodes for providing an energizing signal to the two electrodes; and a controller (250) coupled to the power supply for causing the power supply to energize the two electrodes, thereby attracting the wafer to the wafer support once a wafer has been placed on the wafer support; characterized: by capacitive sensing circuitry (114) electrically coupled to the two electrodes for generating an indicating signal corresponding to a capacitance between the two electrodes; and in that said controller (250) includes means coupled to the capacitive sensing circuitry (114) and receiving the indicating signal corresponding to a sensed capacitance, for monitoring to determine if a wafer is present on the wafer support and to determine if the wafer is properly secured to the wafer support, the controller initiating ion implantation of the semiconductor wafer after the indicating signal indicates that the wafer has been properly secured to the wafer support and suspending ion implantation in the event that the indicating signal indicates that the wafer is not properly secured to the wafer support. Apparatus according to claim 5, further comprising robotic means (254) coupled to the controller (250) for re-orienting the wafer support (10) and moving said wafer (12) into an ion implant orientation, said controller including means for deactivating the robotic means if the indicating signal indicates that the wafer is not properly secured to the wafer support. Apparatus according to claim 5 or claim 6, wherein the capacitive sensing circuitry (114) comprises an oscillator circuit (120) having first and second inputs connected to respective ones of the two electrodes (22,24) for providing an oscillator output whose frequency changes as the capacitance between the electrodes changes, and output circuitry for converting the oscillator output to said indicating signal, the indicating signal based on the frequency of the oscillator output. Apparatus according to claim 7, wherein said output circuitry of the capacitive sensing circuitry (114) comprises an integrator circuit (154) coupled to the oscillator circuit (120) for converting the oscillator output to a voltage level (FVOUT).

Inventors: BLAKE JULIAN GASKILL, HOLDEN SCOTT CARLETON, STONE DALE KEITH, TU WEILIN, BLAKE, JULIAN GASKILL, HOLDEN, SCOTT CARLETON, STONE, DALE KEITH, TU, WEILIN
Applicants: EATON CORP, EATON CORPORATION
Application year: 1994
Main classification: H01L21
IPC codes: B23Q3, H01L21, G01R27